This invention relates to a perpendicular magnetic recording medium adapted to be mounted in a perpendicular magnetic recording type hard disk drive.
The information-oriented society in recent years has continued the rapid advance and the information recording capacity exceeding 80 Gbytes has been required per 2.5-inch magnetic disk in magnetic recording apparatuses represented by HDDs (hard disk drives). In order to satisfy such a requirement in the magnetic disk, it is necessary to realize an information recording density exceeding 133 Gbits per inch2 (133 Gbits/inch2).
Since high recording resolution can be obtained, the perpendicular magnetic recording system composed of a perpendicular two-layer medium having a soft magnetic layer and a single-pole head has been studied and developed as a next-generation high-density recording system.
In such a perpendicular magnetic recording system, it has been an aim to reduce noise caused by the soft magnetic layer. For example, in Japanese Unexamined Patent Application Publication (JP-A) No. S61-5428 (Patent Document 1), determining that noise is mainly caused by a soft magnetic layer, the directions of domain walls and easy magnetization axes of the soft magnetic layer are aligned with the recording track direction, thereby improving S/N, Further, Japanese Unexamined Patent Application Publication (JP-A) No. 2003-187413 (Patent Document 2) describes that domain walls are formed due to the thickness of a soft magnetic backing layer and spike noise caused by reading, with a head, leakage magnetic flux from the domain walls is a problem to be solved. This Patent Document 2 discloses a perpendicular magnetic recording medium that can easily orient easy magnetization axes of the soft magnetic layer (soft magnetic backing layer) in a certain direction, which has conventionally been carried out, so as to eliminate the domain walls that cause the spike noise. That is, a ferromagnetic layer is formed on a substrate using a material in which easy magnetization axes are oriented in the radial or circumferential direction and, further, an antiferromagnetic layer is formed between the ferromagnetic layer and the soft magnetic layer. It is described that this makes it possible to give a radial magnetic anisotropy to the soft magnetic backing layer without applying thereto a heat treatment in a magnetic field or a treatment during magnetization.
Not only in the techniques of the foregoing Patent Documents 1 and 2, but in magnetic recording media having a soft magnetic layer, it has been the aim to reduce noise caused by the soft magnetic layer and it has been an established theory (common sense) that, in order to achieve the aim, easy magnetization axes of the soft magnetic layer should be aligned in a fixed direction. Particularly, in the case of a substrate like a magnetic disk, it has been believed that aligning easy magnetization axes of a soft magnetic layer radially in radial directions from the center of the substrate is the most desirable state for preventing generation of spike noise caused by domain walls of the soft magnetic layer. This is because if a domain wall, i.e. a region where magnetization is inverted, exists in the circumferential direction, a reproducing head detects this inversion of magnetization upon passing through this region, which becomes spike noise. On the other hand, even if a region, where magnetization is inverted, exists in the radial direction, since this differs from the detection direction of the reproducing head by 90 degrees, the head cannot detect it and thus no noise is caused. Therefore, it has been considered best to align the easy magnetization axes of the soft magnetic layer radially in the radial directions from the center of the substrate.
As described above, in the magnetic disk, the magnetic anisotropy is provided radially in order to reduce noise such as spike noise caused by the soft magnetic layer. In order to provide such a magnetic anisotropy, a process is provided wherein, in the state where the substrate is heated after the formation of the soft magnetic layer, the directions of easy magnetization axes are aligned in the radial directions while applying external magnetization in point symmetry with respect to the center of the substrate, a complicated process such as a heat treatment or application of a bias is carried out, or additional layers are formed for aligning the directions of easy magnetization axes like in the technique described in Patent Document 2. Therefore, since process chambers and mechanisms should be prepared for heating the substrate and applying the particular magnetic field and a lot of time is required, there arise problems such as a reduction in productivity/an increase in cost. Particularly, when soft magnetic layers are simultaneously formed with respect to a plurality of small-diameter substrates (disk bases), control of the magnetic field becomes complicated and hence the problems of reduction in productivity/increase in cost are made greater.